By Jan H. Schut
Someone to spin straw into gold, or in this case lignin into plastic. Government money pours into public universities and research centers in places with forestry industries in Canada, Finland, Sweden, Germany, and the northern U.S.—including $120 million worth of recent grants from the U.S. Department of Agriculture for cellulosic biobuels and high value applications for lignin in plastics.
Lignin is the second largest natural biopolymer on the planet after cellulose, wood being roughly 15-35% lignin depending on species and the rest cellulose and hemicellulose. One hundred billion lb/year of lignin is separated globally as a byproduct of wood pulping. Only 2% of that is sold commercially, however, for things like stabilizers in asphalt and dispersants in concrete and textile dyes; 98% is burned internally by pulp mills for energy. An infinitesimally small amount of the commercial 2% is used in plastic, mostly thermosets and adhesives.
Why so little? Lignin is a natural polyphenol, so it can be substituted for some phenolics in thermosetting formaldehyde resins and for some polyols in epoxies and polyurethanes. It’s inherently anti-fungal and improves U.V. resistance. But phenols can slow down reaction time, so typically only 10-15% can be added. Color is also an issue. Industrial lignins are relatively low purity and typically brown, though they can be light and can be bleached. Lignin can also be added to thermoplastic polyurethanes.
Adding it to other thermoplastics, however, is harder. It can only be done if you have thermoplastic lignin, and not much thermoplastic lignin is commercially available. Thermoplastic lignin must have a specific molecular weight, purity, solubility, and thermoplasticity. It can be made by “fractionation” of lignin into distinct molecular weight fractions, mostly by filtration, or by chemical modification, such as acetylation. Isolation from pulping liquors can also be done for higher purity lignin. Otherwise lignin’s branched structure with methoxyl and aliphatic hydroxyl groups, plus chemical residues from pulping, make it unusable.
Only one company in the world has ever successfully commercialized lignin-based thermoplastic compounds—Tecnaro GmbH, Ilsfeld, Germany (www.tecnaro.de), a spinoff from the Fraunhofer Institute for Chemical Technology in Pfinztal, Germany (www.ict.fraunhofer.de) in 1998. Tecnaro developed a patented process (U.S. Pat. # 6509397) for compounding lignin with natural fibers into 100% biobased thermoplastic resins. Its Arboform resins combine lignin with oils, waxes, natural fibers and other biobased additives to increase impact strength and are mainly for injection molding, but also for extrusion, calendaring, thermoforming and compression molding. Its Arboblend resins combine biopolymers like lignin, starch, biopolyesters, cellulose, biopolyamide and biopolyolefins and are for film extrusion.
NEW LIGNIN-CONTENT PLASTICS
Recently, however, three new lignin-content plastic technologies have appeared. Two are thermoplastic; one is thermoset. Two are going commercial, which is astonishing given the difficulties and limited supply of usable lignin. Oak Ridge National Laboratory in Oak Ridge, Tenn. (www.ornl.gov) presented a new lignin-content elastomer technology for the first time at the Society of Plastics Engineers’ ANTEC Conference last May. Oak Ridge’s technology creates high molecular weight lignin, then blends it with functionalized PBD to make thermoplastic elastomer copolymers. Oak Ridge staff researcher Amit Naskar says the patent-pending technology creates a two-phase morphology over a range of blends with 10%-50% lignin.
In Oak Ridge’s process lignin with a weight average molecular weight of about 1000 g/mol is washed in solvents to remove low molecular weight fractions, yielding lignin with about 10,000 g/mol, which is cross-linked with formaldehyde to reach about 30,000 g/mol. This high molecular weight lignin is compounded with soft oligomers to make melt-processable, two-phase copolymers. The soft segments are dicarboxy-terminated, so they react with natural hydroxyl groups on the lignin to improve impact strength. Cross-linked lignin hard segments, which have no detectible Tg, are bridged with low Tg soft segments, making the copolymers melt recyclable. The technology is available for license and could also combine high molecular weight lignin with biopolyesters to make 100% biobased resins.
Another new flexible lignin-content thermoplastic is being developed by a start up venture CycleWood Solutions Inc. in Fayetteville, Ark. (www.cyclewood.com), licensed from the University of Minnesota, St. Paul (www.umn.edu). CycleWood modifies the lignin differently from the method in the patent (U.S. Pat. # 6172204), “which wasn’t commercially feasible,” COO Kevin Oden explains. CycleWood then blends the modified lignin with an undisclosed thermoplastic to make blown film for compostable bags. Scale up work has gone on for a year with the polymer program at the University of Dayton (www.udayton.edu) and made around 300 lb of lignin-content plastic, which has been tested on a lab scale for both cast and blown film. The film is paper bag color and matte finish. It will have to be slightly heavier gauge than conventional HDPE bag film, but will still be less expensive than commercially available compostable materials, Cyclewood’s Oden says. CycleWood, which has won numerous awards for its business plan including an Edison Award, was created by four students in an entrepreneurship class in an MBA program at the University of Arkansas, Fayetteville (www.uark.edu), who found the University of Minnesota technology by searching online. Three of the four partners are also engineers.
Another interesting new development is a lignin-content thermoset. Enerlab 2000 Inc. in St.-Mathieu de Beloeil, Que. (www.enerlab.ca), a maker of PU and PIR foams, has developed lignin-content polyisocyanurate foam and is commercializing it for insulation boards, believed to be a first. Enerlab started the R&D in 2010 as part of a program with the National Research Council of Canada (www.nrc-cnrc.gc.ca). The goal was initially to substitute 10-20% of polyol with lignin, but Enerlab found it could go higher without hurting properties. Enerlab demonstrated the feasibility of making 22% lignin-content foam and is building the industrial process on a pilot scale now. The company expects to commercialize the first renewable-content PIR foam boards early in 2013, says Enerlab president Armand Langlois. Enerlab is extending its technology next to structural insulated panels and spray foam.
LIGNINS ARE COMPLICATED
Different pulping processes produce different lignin types, some water soluble, some not, with different chemical residues. Both water soluble and insoluble lignins can be fractionated for consistent molecular weight and modified for both thermosets and thermoplastics. But water insoluble lignin is easier to use in plastics and more weather-resistant than water soluble lignin. The two biggest wood pulping processes are sulfate (Kraft) and sulfite. Sulfate/Kraft process lignin isn’t water soluble and is mostly burned by pulping mills for energy, except a small amount that is sulfonated for water solubility and sold as emulsifiers and surfactants. Sulphite lignins, made in both acid and alkaline pulping processes, are water soluble and often not burned by mills, but sold as emulsifiers and surfactants.
Two newer pulping processes make high purity lignin which is more promising for plastics. A soda pulping process for straw in India, developed by GreenValue SA in Orbe, Switzerland (www.greenvalue-sa.com), makes high purity lignin that is water insoluble and nearly thermoplastic. Reportedly it can be modified for softening temperatures down to about 130 C. GreenValue is the only large scale commercial source for high purity lignin and sells its products for resins for high-pressure laminates and to several wood-panel mills in India for low-formaldehyde adhesives. GreenValue also sells lignin products for thermoplastics.
Pure Lignin Environmental Technology Ltd. in Kelowna, B.C. in Canada, a 12-year-old company with a patented (U.S. Pat. # 7396434) dilute acid process to separate cellulosics from lignin, also can produce thermoplastic lignins, both water-soluble and insoluble, for use as a filler in plastics, including PP and PE. Pure Lignin says it has sampled numerous test applications and has just licensed its lignin for use at 10-20% in PP. Pure Lignin says 20% lignin in PP and PE improved tensile modulus and flexural modulus by over three times. Its lignin is based on biomass from any vegetation, not just wood.
The great hope for thermoplastic lignin, however, is the so called “organosolv” biorefining process for biofuel. A half dozen biofuel technologies are at the pilot plant stage, but not commercial. The original organosolv biofuel process was developed by a GE Venture Capital company in the ‘80s, called Biological Energy Corp., for alcohol-based wood biorefining. GE intended to convert cellulose into biofuel and sell thermoplastic lignin as a plastic additive, but when oil prices fell, GE backed out and sold the venture to Repap Enterprises Inc., a large Canadian paper company.
Repap wanted the environmentally friendly pulping process, called it Alcell (for alcohol cellulose), and invested $60 million in a demonstration pulping mill. Repap also developed the first lignin-content thermoplastic, putting its patented lignin (U.S. Pat. # 5321065) in powder form (1-5 microns) into LDPE. Repap compounded masterbatches with up to 75% lignin and blew compostable mulch film with 13% lignin. The film was blown in France and the U.S. for sampling. But Repap was heavily in debt for overbuilding paper capacity ahead of a recession and was taken over by a hedge fund and the company broken up.
The Alcell biofuel technology passed to the Canadian government and then to investors who formed Lignol Innovations Ltd., Burnaby, B.C. in Canada (www.lignol.ca). Lignol still has the original GE pilot line and in 2009 built a larger demo plant for high-purity thermoplastic lignin, which Lignol says it has supplied for PP and thermoplastic PU. Because many of the old GE and Repap patents have expired, Lignol requires a “material transfer agreement” with customers to protect its technology.
Historically, the first commercial lignin plastic goes back further than Repap. It was probably a thermosetting adhesive invented by a father/son team in Canada in the 1950s. George Tomlinson Sr. found a way to cook and isolate lignin from sulfonate/alkali paper pulping, then with son George Jr. used the lignin to replace phenolic in thermosetting formaldehyde adhesives. The Tomlinson technology was commercialized by Arborite Co. in Montreal, now a unit of ITW (www.arborite.com), which used lignin in adhesive to make laminated wood kitchen counters.